The present invention relates generally to subscriber line interface circuits for digital telephone switching systems, and more specifically to a subscriber line interface circuit for a switching system serving both ISDN (Integrated Services Digital Network) and POTS (Plain Old Telephone Service) subscribers.
Subscriber line interface circuits are of two types, one for serving subscribers using POTS stations. The other type of SLIC serves subscribers using ISDN stations which employ the echo cancellation scheme to exploit the full bandwidth of the two-wire transmission channel, instead of the time division duplex scheme, or `ping-pong` transmission where the bandwidth is divided between the opposite ends of the transmission channel. The functional separation of the types of subscriber line interface circuit is due to the remarkable differences between the frequency range of the signals for POTS and ISDN lines and their termination impedances. Specifically, the SLIC of the POTS type handles signals in the range of frequencies from 0.3 to 3.4 kHz with a termination impedance varying in the range from 600 to 900 ohms, whereas the ISDN type SLIC handles 2B1Q (two bits one quarternary) line-coded signals in the range from several tens of kHz to more than 100 kHz with a termination impedance as low as 100 ohms. Digital signal processors have also been developed for use with different types of subscriber line interface circuit.
However, needs arise to replace a subscriber line interface circuit designed for POTS stations with one designed for ISDN stations when the subscriber so desires and such needs will increase as ISDN network expands.